Add new navigation hotkeys

[cleac]

This is a proposal, detached from #128 by request of @bleakgrey.

Problem

It is expected from user that application handles keystrokes properly on context. Here some examples of them:

• when user presses Home, the viewport jumps to the top of the list
• when user presses End, the viewport jumps to the bottom of available list
• when user presses Page Up, the viewport jumps about a page up
• etc...

There are more.

Proposal of resolving issue

This issue can easily be fixed applying those expecting moves. Here are things that are expected the most:

• Home -> the viewport jumps to the top of the list and (maybe) new toots are loaded
• End -> the viewport jumps to the bottom of available list and (maybe) older toots are loaded
• Page Up -> the viewport jumps about a page up
• Page Down -> the viewport jumps about a page down
• [x] Tab -> switch the tab
• [x] Shift+Tab -> switch tab in backward direction

[cleac]

By the way, this one corresponds a bit with #132

[bleakgrey]

• Tab is reserved by GTK to switch focus on the visible widgets, so it can't be used. Moreover, this action is already implemented with Alt + 1-4.
• Going back is available too with Alt + Backspace or Alt + <-.

Available keystroke combinations are a bit more obvious now since they are displayed in the tooltips in the master branch.

[cleac]

Oh, that is the reason why most of application have those strange Ctrl+PageUp keystrokes to change tabs :smile: Then Alt+ is the sanest way to do this, thanks for mentioning