Fix UpdateModsList crash on Mono

[HebaruSan]

Problem

I'm getting this crash at GUI startup on Mono:

[xcb] Too much data requested from _XRead
[xcb] This is most likely caused by a broken X extension library
[xcb] Aborting, sorry about that.
cli: ../../src/xcb_io.c:732: _XRead: Assertion `!xcb_xlib_too_much_data_requested' failed.
Stacktrace:

  at <unknown> <0xffffffff>
  at (wrapper managed-to-native) System.Windows.Forms.X11Keyboard.XCreateFontSet (intptr,string,intptr&,int&,intptr) [0x0000b] in <a78030b9a9a543b381a52822d20656d0>:0
  at System.Windows.Forms.X11Keyboard.CreateOverTheSpotXic (intptr,intptr) [0x00023] in <a78030b9a9a543b381a52822d20656d0>:0
  at System.Windows.Forms.X11Keyboard.CreateXic (intptr,intptr) [0x00042] in <a78030b9a9a543b381a52822d20656d0>:0
  at System.Windows.Forms.X11Keyboard.CreateXicForWindow (intptr) [0x00000] in <a78030b9a9a543b381a52822d20656d0>:0
  at System.Windows.Forms.X11Keyboard.FocusIn (intptr) [0x00032] in <a78030b9a9a543b381a52822d20656d0>:0
  at System.Windows.Forms.XplatUIX11.SetFocus (intptr) [0x00054] in <a78030b9a9a543b381a52822d20656d0>:0
  at System.Windows.Forms.XplatUI.SetFocus (intptr) [0x00000] in <a78030b9a9a543b381a52822d20656d0>:0
  at System.Windows.Forms.Control.Select (System.Windows.Forms.Control) [0x0005d] in <a78030b9a9a543b381a52822d20656d0>:0
  at System.Windows.Forms.Control.FocusInternal (bool) [0x0002a] in <a78030b9a9a543b381a52822d20656d0>:0
  at System.Windows.Forms.Form.FocusInternal (bool) [0x00016] in <a78030b9a9a543b381a52822d20656d0>:0
  at System.Windows.Forms.Control.Focus () [0x00000] in <a78030b9a9a543b381a52822d20656d0>:0
  at CKAN.Main.SwitchEnabledState () [0x0003e] in <d26c8e7f3da1413293cb83e9482c631b>:0
  at CKAN.Main._UpdateModsList (bool,System.Collections.Generic.IEnumerable`1<CKAN.ModChange>) [0x005aa] in <d26c8e7f3da1413293cb83e9482c631b>:0
  at CKAN.Main/<>c__DisplayClass256_0.<UpdateModsList>b__0 () [0x0001b] in <d26c8e7f3da1413293cb83e9482c631b>:0
  at System.Threading.Tasks.Task.InnerInvoke () <0x0004e>
  at System.Threading.Tasks.Task.Execute () <0x00024>
  at System.Threading.Tasks.Task.ExecutionContextCallback (object) <0x00028>
  at System.Threading.ExecutionContext.RunInternal (System.Threading.ExecutionContext,System.Threading.ContextCallback,object,bool) <0x00171>
  at System.Threading.Tasks.Task.ExecuteWithThreadLocal (System.Threading.Tasks.Task&) <0x000b2>
  at System.Threading.Tasks.Task.ExecuteEntry (bool) <0x0010f>
  at System.Threading.Tasks.Task.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem () <0x00007>
  at System.Threading.ThreadPoolWorkQueue.Dispatch () [0x00074] in <7b0d87324cab49bf96eac679025e77d1>:0
  at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback () <0x00005>
  at (wrapper runtime-invoke) <Module>.runtime_invoke_bool (object,intptr,intptr,intptr) [0x0001e] in <7b0d87324cab49bf96eac679025e77d1>:0

Cause

SwitchEnabledState is running on the wrong thread. It needs to run on the GUI thread, but instead it's running on the background thread handling the registry load.

Changes

Now the usual Util.Invoke call is used to put SwitchEnabledState on the GUI thread.

Fixes #2624.