Font: Overhaul font asset discovery

Previously, any font names requested by RGSS would be translated
directly to filenames by lowercasing and replacing spaces with
underscores (and finally doing some extension substitution).
To make this whole thing work smoother as well as get closer to
how font discovery is done in VX, we now scan the "Fonts/" folder
at startup and index all present font assets by their family name;
now, if an "Open Sans" font is present in "Fonts/", it will be
used regardless of filename.

Font assets with "Regular" style are preferred, but in their
absence, mkxp will make use of any other style it can find for
the respective family. This is not the exact same behavior as
VX, but it should cover 95% of use cases.

Previously, one could substitute fonts via filenames, ie. to
substitute "Arial" with "Open Sans", one would just rename
"OpenSans.ttf" to "arial.ttf" and put it in "Fonts/". With the
above change, this is no longer possible. As an alternative, one
can now explicitly specify font family substitutions via mkxp.conf;
eg. for the above case, one would add

fontSub=Arial>Open Sans

to the configuration file. Multiple such rules can be specified.

In the process, I also added the ability to provide
'Font.(default_)name' with an array of font families to search
for the first existing one instead of a plain string.
This makes the behavior closer to RMXP; however, it doesn't
work 100% the same: when a reference to the 'Font.name' array is
held and additional strings are added to it without re-assignig
the array to 'Font.name', those will be ignored.

mkxp.conf.sample

@@ -113,3 +113,18 @@
 # RTP=/path/to/rtp1
 # RTP=/path/to/rtp2.zip
 # RTP=/path/to/game.rgssad
+
+
+# Font substitutions allow drop-in replacements of fonts
+# to be used without changing the RGSS scripts,
+# eg. providing 'Open Sans' when the game thinkgs it's
+# using 'Arial'. Font family to be substituted and
+# replacement family are separated by one sole '>'.
+# Be careful not to include any spaces.
+# This is not connected to the built-in font, which is
+# always used when a non-existing font family is
+# requested by RGSS.
+# (default: none)
+#
+# fontSub=Arial>Open Sans
+# fontSub=Times New Roman>Liberation Serif