Add option to hide title bar on linux

[elliotnash]

This PR adds the ability to complete hide the title bar on linux. This replaces the Use Custom TitleBar option with a TitleBar Style selection with the following options:

Native	Custom	Hidden

[jjoelj]

~~Hey, can you change the base branch to joel/desktop/1.15/1.15.5.0~~ Nvm I can do that

Also, what's the use case for this?

[wiggleforlife]

Also, what's the use case for this?

Looks like they're on a tiling window manager, where no apps have title bars. Instead, keyboard shortcuts are used to manage windows. BlueBubbles would most likely be the only app that still has a bar in this scenario.

[elliotnash]

Sorry I missed this!

Yeah, the use case is tiling window managers where application titlebars are hidden and the compositor provides its own minimal decorations. In these setups, the compositor can advertise a preference for server‑side decorations and inform clients that they are tiled, but Flutter/bitsdojo_window currently forces client‑side decorations in native mode and does not obey that hint.

For example, on my system all window control buttons are disabled; with useCustomTitlebar off, bitsdojo_window still reserves and draws an empty titlebar area above the app content (the left image). A "hidden" option for the titlebar lets the app surface be fully decoration‑less so the compositor’s own decorations can be the only visible ones, which is the expected behavior on tiling WMs.