Windows: Fix wrong 16x16 icon in window frame.

wxIconBundle unexpectedly doesn't load all .ico sizes from a Windows
resource entry.  To workaround this, install Poedit.ico as a file and
load that.

Fixes the icon associated with Poedit's windows to provide both 32px and
16px versions, as needed for proper display in both the titlebar and
task switcher.
OS X: never modify text when setting it into translation text control.

wxTextCtrl implementation on OS X uses insertText:, which is intended
for user input and performs some user input processing, such as
autocorrections.  We need to avoid this, because Poedit's text control
is filled with data when moving in the list control.  Solve this by
using a customized control with overriden DoSetValue().

Fixes #81.