Undo/Redo shortcuts when focus is in various dialogs

[windymilla]

Notes:

1. When focus is in the list of a checker dialog (or WF), Cmd/Ctrl+Z causes an Undo in the main text window.
2. Page Sep dialog has its own Undo/Redo buttons and shortcuts are bound to those.
3. Other dialogs (e.g. Convert Text Markup) generally ignore undo/redo shortcuts, so user has to change focus to main text window to use keyboard shortcut.
4. Other shortcuts, such as Cmd/Ctrl+S (for Save) do work when focus is in another dialog
5. Other "global" shortcuts, such as Cmd/Ctrl+C, X, V (Copy, Cut, Paste) work in the dialog with focus, e.g. you can cut/paste in the Search entry field of the SR dialog.
6. Undo/Redo is an oddity, because it's global in the sense of being built-in to the main text (and the read-only text lists in checkers/WF), but by default does nothing in the simple entry fields (like Search in S/R). Also, many dialogs "do something" to the main text, which it is reasonable to want to "undo"/"redo".

Questions:

1. Do we want the undo/redo shortcuts to automatically undo/redo maintext window changes when typed in a dialog, or is it preferable that the user transfers focus to the main window, which puts the responsibility back on then to check that the undo actually did what they expected?
2. Do we want undo/redo to work in all dialogs, or just specific ones like Convert Text Markup that make significant changes?
3. What about the undocked Image Dialog (currently does not undo/redo)?
4. What is "modern" behavior? In my experience, some modern applications either have the "dialog" as part of their main window, e.g. Search in Visual Studio, or the dialog is "modal" (i.e. you can't do anything else but interact with the dialog until you close it). Some applications, e.g. Notepad++, have a separate Search dialog, but undo when focused on that does not undo the last change in the main text - it can however undo characters that are typed into the Search field.
5. FWIW, in GG1, there are very few places you can use the Undo shortcuts from - the main text window and Page Sep Fixup are the only two that come to mind. Do we want that behavior in GG2, or is a partial/complete support of Undo a better idea?