Workspaces, tabs, and multi-monitor support

[Stebalien]

EXWM currently supports "workspaces" by creating a new frame per workspace. Additionally, multiple monitors can be supported by assigning workspaces to monitors.

Unfortunately:

1. Assignment of workspaces to monitors isn't automatic.
2. With the introduction of the tab-bar, frame-based workspaces are mostly redundant.

So, my proposal is to:

1. Remove or deprecate the current workspace feature.
2. Associate floating windows with tabs, unless they're marked as sticky. We may have to advise tab-bar-select-tab, but I'm hoping that the window-configuration-change-hook is enough.
3. Make the randr module automatically create/delete frames.

The tricky parts will be:

1. Making this "just work", especially monitor configuration.
2. Backwards compatibility. We can probably introduce some aliases for, e.g., exwm-workspace-next and friends, but we likely won't be able to replicate all features.

The upsides are:

1. Better integration with Emacs packages by using the tab-bar.
2. Visual indication of the current workspace (again, via the tab-bar).
3. Automatic support for multiple monitors.