Add new multi-select checkboxes to message list

Use the checkboxes of AOSP's Email app.
Use the color chip area as click target for the multi-select checkboxes
Make the color chip area slightly smaller
Added explicit width and height for the new message list checkbox

Without this change Android 2.x does weird things.
better guarding for the  folders menu item in the face of search folders with multiple accounts
Fix "Folders" menu item and hotkey handling in MessageList
Fix code to reset the database when upgrading fails

Previously the app crashed when upgrading the database failed. Now we
reset the database version and run the upgrade code again (recreating
all tables).