Add browse buttons for selecting cygwin/MSYS socket paths.

Issue dlech/KeeAgent#84
Expand environment variables in path before creating cygwin/msys socket.

Issue dlech/KeeAgent#85
remove TODO since v2.17 is already over 10 versions (3 years) old.
Add column to entry list show if SSH key is loaded or not.

Issue dlech/KeeAgent#43
Issue dlech/KeeAgent#81
Add keyboard shortcuts for loading keys.

This is done though the password entry context menu. CTRL+K will load a key.
CTRL+SHIFT+K will load a key and then open the URL from the entry. Also,
double-clicking the custom column to load a key calls back to the KeeAgentExt
AddEntry function so that constraints will be properly loaded as well.

Issue dlech/KeeAgent#81
Add lifetime constraint to password entry settings.

Also fix cross threading crash in KeeAgentColumnProvider.

Issue dlech/KeeAgent#81