Windows: Don't clear first translation with custom font.

Due to a bug in wx, setting custom fonts on the text field would emit
wxEVT_TEXT event, causing Poedit to mistakenly clear the translation.
Proper fix will be on wx trunk, but not 3.0, so work around it here.

Fixes #589.
Don't assert on empty src search path.

Ignore them when scanning.  When loading a catalog or setting
properties, filter empty entries out.
Don't show individual per-catalog merge summaries in the manager.
OS X: Yet another attempt to fix progress info updates.
Don't show error messages if the user cancelled update from src/POT.

Pressing Undo should not result in errors about failure to update.
Don't autotranslate after failed or cancelled update.
Ask to save changes before doing update from src/pot.

This is not strictly necessary, but it's a useful precaution against the
user accidentally doing a bad update and loosing all unsaved data.  This
way, there will be at least a saved version to revert to.