Documented configuring the main window

Created a new page to document Moving, sliding, tabbing and locking the panes.

Files changed:

index.rst <- add new menu item 'Configuring the OpemLP Main Window'

configure.rst <- add link View-Lock Panels

menu_items.rst <- add link to View-View

Files Created:

configure_ui.rst

pics/config_ui_blue_placement.png

pics/config_ui_tabbed.png

pics/config_ui_tab_blue_placement.png

[Bug #910350]