add tags/workspaces

[BKSalman]

every monitor will have their separate tags/workspaces.

could look like this:

struct WM {
    tags: Vec<Tag>,
}

struct Tag {
    id: u32,
    name: String,
    layout_manager: LayoutManager,
    // other fields
}

or like this:

struct WM {
    tags: Vec<Tag>,
    layout_manager: LayoutManager,
}

struct Tag {
    id: u32,
    name: String,
    // other fields
}

maybe I would like to pin one of the tags to a specific monitor, but that's for later

TODOs:

• add tags
• change the _NET_NUMBER_OF_DESKTOPS property
• track the _NET_CURRENT_DESKTOP property
• unmap windows that are on other tags ( not sure if that's how you hide the windows from other tags )
• map windows on the current tag