Fix reinstall prompt crash

[HebaruSan]

Problem

My Mono does a hard crash when CKAN tries to ask the user whether to reinstall a changed mod after a refresh:

	  at <unknown> <0xffffffff>
	  at System.Windows.Forms.X11Keyboard:XCreateFontSet <0x000ce>
	  at System.Windows.Forms.X11Keyboard:CreateOverTheSpotXic <0x000e7>
	  at System.Windows.Forms.X11Keyboard:CreateXic <0x00117>
	  at System.Windows.Forms.X11Keyboard:CreateXicForWindow <0x00053>
	  at System.Windows.Forms.X11Keyboard:FocusIn <0x000c3>
	  at System.Windows.Forms.XplatUIX11:SetFocus <0x000ef>
	  at System.Windows.Forms.XplatUI:SetFocus <0x0003c>
	  at System.Windows.Forms.ContainerControl:SendControlFocus <0x000bf>
	  at System.Windows.Forms.Form:SetVisibleCore <0x004f3>
	  at System.Windows.Forms.Control:set_Visible <0x0004b>
	  at System.Windows.Forms.Control:set_Visible <0x00083>
	  at System.Windows.Forms.Application:RunLoop <0x003df>
	  at System.Windows.Forms.Form:ShowDialog <0x008f7>
	  at System.Windows.Forms.Form:ShowDialog <0x0002b>
	  at CKAN.YesNoDialog:ShowYesNoDialog <0x001cf>
	  at CKAN.YesNoDialog:ShowYesNoDialog <0x000a3>
	  at CKAN.Main:YesNoDialog <0x00057>
	  at CKAN.Main:YesNoDialog <0x000a3>
	  at CKAN.GUIUser:RaiseYesNoDialog <0x0003f>
	  at CKAN.Repo:HandleModuleChanges <0x001bf>
	  at CKAN.Repo:UpdateAllRepositories <0x00573>
	  at CKAN.Main:UpdateRepo <0x005c3>
	  at System.ComponentModel.BackgroundWorker:OnDoWork <0x00046>
	  at System.ComponentModel.BackgroundWorker:WorkerThreadStart <0x000c2>
	  at System.ComponentModel.BackgroundWorker:<RunWorkerAsync>b__27_0 <0x00033>
	  at System.Threading.Tasks.Task:InnerInvoke <0x0008f>
	  at System.Threading.Tasks.Task:Execute <0x00037>
	  at System.Threading.Tasks.Task:ExecutionContextCallback <0x0005b>
	  at System.Threading.ExecutionContext:RunInternal <0x00191>
	  at System.Threading.ExecutionContext:Run <0x00042>
	  at System.Threading.Tasks.Task:ExecuteWithThreadLocal <0x000f6>
	  at System.Threading.Tasks.Task:ExecuteEntry <0x000dc>
	  at System.Threading.Tasks.Task:System.Threading.IThreadPoolWorkItem.ExecuteWorkItem <0x00026>
	  at System.Threading.ThreadPoolWorkQueue:Dispatch <0x00279>
	  at System.Threading._ThreadPoolWaitCallback:PerformWaitCallback <0x0001c>
	  at <Module>:runtime_invoke_bool <0x00086>

Cause

The update runs on a background thread, and then it uses IUser to do GUI stuff.

Changes

Now YesNoDialog.ShowYesNoDialog does all its GUI operations on the GUI thread and uses a TaskCompetionSource to propagate the result back to the background thread.

I made the parent form a parameter because Util.Invoke wasn't reliably switching to the right thread in my testing when the first parameter was from the YesNo form. I think that parameter needs to be a UI object that's properly connected to the rest of the app. As a bonus, this means we can pass the parent form to ShowDialog.

I also realized that the if you had a ShowYesNoDialog call with the custom button caption parameters followed by a call without them, the buttons would keep the custom captions from the first call, so they need to be reset each time.