Functionality and UI overhaul #1

Added a ton of features:

The title, rather than reading "Would you like to try or install Lubuntu?" now reads "Lubuntu: Welcome to the Next Universe", which I feel fits better since that's the "slogan" or whatever you call it on the website.
The title has been moved down and the buttons moved up, which made things look a bit better IMO.
A background has been placed behind the title to make it easily readable.
Tooltips with explanatory info have been added to the buttons - the tooltips have been themed to look nice with the rest of the installer prompt.
Rather than autostarting when LXQt starts, the installer prompt is now its own X session, which starts independently from LXQt.
Clicking "Try Lubuntu" will now start LXQt after the installer prompt is closed.
When "Install Lubuntu" is pressed, the buttons disappear before the installer launches - this avoids them possibly getting pressed during the installation process.
If Calamares crashes or closes, the installer prompt will close and start LXQt.

A lot of these changes are opinion-based, but I feel that they all work together to make the installer prompt look and function a lot better. I'm happy to (at least try to) implement any changes necessary.

A video of this version of the installer prompt in action: https://imgur.com/a/HSxvC7x (Note that this is from before the tooltip fix was applied, so if the tooltips look a bit weird, that's why.)