Add actions for switching to a specific tab

Summary:
Add actions to switch to each of the first 9 tabs and another action to
switch to the last tab.

This feature makes it much easier to quickly switch between tabs just
like you normally would be able to when using a web browser or other
applications.

Reviewers: #vdg, #dolphin, ngraham, elvisangelaccio

Reviewed By: #vdg, #dolphin, ngraham

Subscribers: meven, ngraham, elvisangelaccio, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D24353