Add Filelight menu to Tools menu

!

The menu for installing and launching Filelight was only available through the status bar/"Free space info" button until now. By default we do no longer show the "free space info" button because the status bar has been changed to be smaller and show less information by default.

This commit adds the "Manage Disk Space Usage" menu to the "Tools" menu in the menu bar so it keeps being available to users even if they use the smaller default status bar.

There is one weird bug I found: When accessing the "Install Filelight" menu through the hamburger "More" menu it works the first time, but after completely closing the hamburger menu once, the "Install Filelight" menu will no longer appear/will appear like empty. This is no longer an issue when Filelight has been installed, then everything works in the "More" menu the same way it worked everywhere else. I don't think this is a blocker because a user is unlikely to repeatedly visit the "Install Filelight" menu from the "More" menu without triggering the installation.

I extracted the menu into a new class for this and then wired it up from the main window/menu bar the same way it is wired up from the status bar.

cc @akselmo