Makefile.am: add -lm for Math.floor() issue on my development platform

The issue: undefined reference to symbol 'floor@@GLIBC_2.2.5'

Appears to be related to to pkg-config not including libm for glib,
but may be outdated vala bindings or similar.
src/Makefile.am: switch to builtin vala support in autotools
move gtksourceview out of src directory
add and use RuntimeEnvironment helper class

to support different behavior depending on context.
add some debugging code to help detect leaking layers and menu items
rewrite appearance customization feature

* adds Selected Item Background and Header/Footer colors
* removes browser preview layer in favor of:
** keeping the browser partially visible via menu overlay
** updating the visible browser dyamically
appearance: make browser item spacing configurable
update default appearance (bar selection style)
tweak appearance parsing/editing logic
second appearance customization feature rewrite

adds
* customization of menu appearance
* appearance presets/themes

Some bugs and debugging code still present. Committing before adding new behaviors/features.
MenuAppearance: only allow monospaced fonts to be selected
MenuMessageLayer: size text to best fit
third appearance customization feature rewrite

Splits up appearance editing into multiple "areas", to allow more complex appearance definitions.

Currently this adds support for distinct font and text color in header/footer of browser and menu.
make browser list spacing and menu border color editable
add appearance debugging convenience commands
Selector: truncate rendered item text to fit in configured item space
tweak menu controls for consistency
update default appearance
ship old default appearance
move default appearance constants to AppearanceDefaults class