DisplayConfig: Add support for interlaced modes

Differentiate between non-interlaced and interlaced modes. This is done
by appending an "i" after the resolution part of the mode ID, and
adding a 'is-interlaced' (b) property to the mode properties.

https://bugzilla.gnome.org/show_bug.cgi?id=765011

src/tests/monitor-store-unit-tests.c

@@ -36,6 +36,7 @@ typedef struct _MonitorTestCaseMonitorMode
   int width;
   int height;
   float refresh_rate;
+  MetaCrtcModeFlag flags;
 } MonitorTestCaseMonitorMode;
 
 typedef struct _MonitorTestCaseMonitor