LibGUI: Mechanism to access off-window tabs

[linusg]

Currently there seems to be no way (GUI, mouse, keyboard) to access tabs that are "off-window" - i.e. if there are more tabs than horizontally fit into the window.

~~This is a huge issue for tab hoarders, as the browser in its default size only allows four tabs to be displayed! :^)~~

Edit: I just remembered that we do have Ctrl+Pg{Up,Dn} in the Browser, but that still affects something like SystemMonitor.

Some ideas:

• Scrolling on the tab buttons moves them left and right (Firefox does this)
• Small < and > buttons on the left and right of the tab button row if there are more tabs on the left or right (not sure what software does this but I've definitely seen it before)

[AtkinsSJ]

Other options:

• A drop-down menu on the right of the tab bar if there are additional tabs. (Firefox has this as well as the scrolling; CLion) We do something similar already for the bookmarks bar.
• Split the tab bar onto multiple rows. (Windows) Not saying it's a good option, but it is an option. Better suited to fixed sets of tabs, it'd be horrible in a browser.

[linusg]

A drop-down menu on the right of the tab bar if there are additional tabs

I like this!

Split the tab bar onto multiple rows

I hate this :D