Increase TXMaximumIRCUsernameLength from 20 to 40.

This will fix issues for bitlbee users who connect to xmpp servers that
generate long usernames.
WARNING: THIS COMMIT BREAKS MAVERICKS SUPPORT (for now)

Summary of commit:

To support both Yosemite and Mavericks, the main window was split into
two separate window files.

TXCMenuController.xib has two responsibilities now. It creates fresh
instances of TXMasterController, TXMenuController, and also sets up all
application menus.

TVCMainWindow*.xib is now responsible for the main window itself. This
file is loaded inside the -init of TXMasterController then the
-awakeFromNib method inside the master controller is still responsible
for app delegation.

This commit breaks Mavericks support because the main window file for
it is empty. That has not been converted over to the new format yet.

The focus of the commit was to finish work on the Yosemite version of
the text field. It now supports both dark and light variants.

For debugging purposes, there is code that can be commented at the top
of TVCMainWindowTextView.m in its -awakeFromNib method to switch
between dark and light mode.
Updated view constraints
Member list inherits default appearance
Merge pull request #465 from eatnumber1/master

Increase TXMaximumIRCUsernameLength from 20 to 40.
Fix appearance of text field on developer preview 2
Text field fixes
comment out dark mode
Improve text field drawing
Do nothing when buttons are hidden
Set code signing ID to match existing certificate