[pgAdmin][RM2653] Lock layout

Hi Hackers,

Attached is the patch to introduce feature to lock the layout with three
different levels:
1) None - No locking
2) Prevent docking - The docking/undocking of panels will be disabled
3) Full lock - The resizing along with docking/undocking of panels will be
disabled

- This can be changed from new menu under File->Lock layout-><The level> or
from Preferences->Browser->Display->Lock Layout
- JS test cases added.

With this patch I have also changed below:
1) Moved the maintenance dialog button group control to new common control
named RadioModern, which I have re-used in preferences for lock layout
levels.
2) Added main nav bar menu with check feature, the menus which can checked.
3) Ability to add submenus from existing get_own_menuitems in python code
which is not possible currently.
3) Separated all the layout related functions from browser.js to new file
layout.js to help JS testing.

Kindly review.

--
Thanks and Regards,
Aditya Toshniwal
Software Engineer | EnterpriseDB India | Pune
"Don't Complain about Heat, Plant a TREE"