Add flag to emulate mobile during preview

Since the is_mobile check was moved to compile-time instead of runtime
in 0a6a441041, we need a way to override
the check when previewing using the 'mobile' flag.

Signed-off-by: kernelkind <kernelkind@gmail.com>
Signed-off-by: William Casarin <jb55@jb55.com>

Cargo.toml

@@ -45,6 +45,7 @@ egui_virtual_list = "0.3.0"
 
 [features]
 default = []
+emulate_mobile = []
 profiling = ["puffin", "puffin_egui", "eframe/puffin"]
 
 [profile.small]