redesign tile action menu with toolbar and submenus

- replace old PopupMenu (flat preset list) with PopupPanel-based menu
- add 3-icon toolbar: ⚙ Settings, ℹ Info, ✕ Remove (+) + Add
- red ✕ on tiles removes the tile; green + on empty space opens add-tile submenu
- 'Themes ▸' button opens a submenu with all ShaderPresets
- add-tile submenu populates from PluginManager.get_all_tile_defs()
- all toolbar buttons are 48×48 flat buttons with 24px font for touch
- orphan PopupPanel children cleaned up during grid rebuild

config/layouts/Main.cfg

@@ -1,7 +1,7 @@
 [layout]
 
 name="Main"
-saved_at="2026-05-21T09:09:37"
+saved_at="2026-05-21T09:16:49"
 
 [tiles]
 
@@ -10,7 +10,7 @@ count=3
 [tile_0]
 
 id="system_monitor/cpu"
-col=0
+col=1
 row=0
 w=1
 h=1
@@ -18,7 +18,7 @@ h=1
 [tile_1]
 
 id="system_monitor/memory"
-col=1
+col=2
 row=0
 w=1
 h=1
@@ -26,7 +26,7 @@ h=1
 [tile_2]
 
 id="system_monitor/testing"
-col=2
+col=0
 row=0
 w=1
-h=2
+h=3