Add perMonitor mode to <margins> for per-monitor gap support

[dubitabam]

Problem

The existing element applies a global strut across the entire desktop. On multi-monitor setups, left/right margins only affect the outermost screen edges, making it impossible to add consistent insets per monitor.

Solution

A new perMonitor attribute on the existing element:

<margins perMonitor="yes">
    <top>8</top>
    <left>8</left>
    <right>8</right>
    <bottom>8</bottom>
</margins>

When perMonitor="yes", margins are no longer added as global struts. Instead they are applied per monitor inside screen_area() when a specific monitor head is requested, so they naturally propagate to all consumers:

Maximize (MaximizeFull, MaximizeVert, MaximizeHorz)
Snapping (MoveResizeTo, MoveToEdge)

Not affected: fullscreen windows (screen_physical_area_monitor is used there), floating windows. Panel struts (e.g. Polybar) are respected - they and the margins stack correctly.

The default (perMonitor="no") preserves existing behaviour exactly.

Changes

• config.h/c - gboolean config_margins_per_monitor, read from the perMonitor attribute via obt_xml_attr_bool, defaults to FALSE
• screen.c - margins skipped as struts when perMonitor="yes"; instead applied in screen_area() inside the if (head == i) block after monitor clamping
• client.c - MoveToEdge initial fallback edge adjusted to the margin position when perMonitor="yes"
• data/rc.xml - sample config updated with perMonitor attribute and documentation

Testing

Tested with Xephyr (single monitor) and a dual-monitor setup with Polybar strut on the bottom.

[dubitabam]

Fix: Move per-monitor margin application from screen_area() to specific call sites

While testing the initial implementation, I discovered a side effect: applying margins inside screen_area() affected all callers of that function, not just maximize and snap operations. This caused problems for any app that uses screen_area() indirectly, most notably drop-down terminals like Guake that call window.maximize() via GTK to fill the screen.

Root cause: screen_area() is called from many places: _NET_WORKAREA calculation, window placement (client_manage), client_find_onscreen, and others. By applying margins there, all of those were affected unintentionally.

Fix: Remove margins from screen_area() and apply them explicitly at the three call sites that should respect them:

• client_configure_full(): maximize path (max_horz/max_vert)
• moveresizeto.c run_func(): MoveResizeTo action
• client_find_edge_directional(): per-monitor areas for MoveToEdge

Margins now only affect explicit user-initiated maximize and snap actions, while floating windows, _NET_WORKAREA, and app-managed window geometry remain completely unaffected.