v1.6pre18

Changes since 1.6pre17:
Added: Ctrl+Up/Down to navigate tabs in tab stack
Added: Ctrl+Alt+U shortcut key for Unstack Tab
Added: Ctrl+Alt+[+/-] shortcut keys for Expand/Collapse Stack
Added: Ctrl+Alt+0 shortcut key for Expand/Collapse All Stacks
Added: Expand/Collapse All Stacks menuitem
Removed: Ctrl+X shortcut key for Copy Page URL and Close Tab
Remvoed: Ctrl+n shortcut keys for Select nth Unpinned Tab
Removed: Ctrl+Up/Down shortcut keys for Previous/Next Group
Removed: Ctrl+Shift+F3/F4/F5 shortcut keys for Close Left/All/Right Tabs
Improved: Remember MRU tab order across sessions