Don't use xpm, but png

Kinda fixes #1

Load times seem to be slightly higher though,
because of png decoding, but this shouldn't
be an issue, since pngs are only loaded
on startup and preference save, not on volume
meter updates.

Cargo.toml

@@ -3,9 +3,6 @@ name = "pnmixer-rs"
 version = "0.1.0"
 authors = ["Julian Ospald <hasufell@posteo.de>"]
 
-[build-dependencies]
-gcc = "^0.3.0"
-
 [dependencies]
 alsa = "^0.1.8"
 alsa-sys = "^0.1.1"
@@ -28,6 +25,7 @@ toml = "^0.4.2"
 which = "*"
 xdg = "*"
 libnotify = { path = "3rdparty/libnotify", optional = true }
+png = "^0.9.0"
 
 [dependencies.gtk]
 git = "https://github.com/gtk-rs/gtk.git"