v2 - add skins, support for Backpack v7 and drop support for v6

[pxpm]

Updates theme tabler for the new Backpack v7 version.

[tabacitu]

What's new

This new version:

• upgrades to the latest Tabler version;
• splits custom CSS into multiple files, so it's easier to override;
• changes a few layout defaults (eg. by default containers are now container-xxl instead of container-xl to show more info on huge screens);
• adds SKINS: small CSS files to add different backgrounds/styling/branding colors;

Skins

Default skins, out-of-the-box:

• ✅ backpack-color-palette - makes primary, secondary, danger, warning colors match Backpack's branding;
• ✅ glass - makes cards semi-transparent;
• ✅ fuzzy-background - adds a colorful modern background;
• 🟩 dotted-background - simple dotted background; works great with glass;
• 🟩 vertical-lines-background - electric vertical-lines background; beautiful dark mode; works great with glass;
• 🟩 pinstripe-background - elegant minimalist background;
• 🟩 paper-background - simple architecture-inspired background;

Notes:

• skins with ✅ above are selected by default upon installation; you can easily unselect a skin or create your own, by publishing the tabler config file using php artisan vendor:publish --tag=theme-tabler-config;
• you can see the skins in action at https://next.backpackforlaravel.com/admin/login - once logged in, click the color-pallete icon in the top-right corner;