app: new default dynamics is "Pressure Size".

Another sensible default change proposed by Aryeom. Until now, all tools
which could make use of dynamics started with "Dynamics Off". This makes
it very hard for beginners to start using GIMP with a tablet directly
without actually knowing the technical settings of the software.
This is even more anticlimactic in GIMP 3 as tablet are supposed to work
from scratch (and they do), but without a proper dynamics, you can't see
it. And same as it used to in GIMP 2.x, at first launch, it looks like
your tablet is not recognized.

We hesitated between "Pressure Size" and "Pressure Opacity". Both are
sensible choices and would directly show your tablet works. Also they
rely entirely on tablet-specific input (pressure), unlike e.g. speed or
direction, which is much better for the purpose of showing the tablet
works and is properly supported.

app/config/gimpcoreconfig.c

@@ -52,7 +52,7 @@
 
 
 #define GIMP_DEFAULT_BRUSH         "2. Hardness 050"
-#define GIMP_DEFAULT_DYNAMICS      "Dynamics Off"
+#define GIMP_DEFAULT_DYNAMICS      "Pressure Size"
 #define GIMP_DEFAULT_PATTERN       "Pine"
 #define GIMP_DEFAULT_PALETTE       "Default"
 #define GIMP_DEFAULT_GRADIENT      "FG to BG (RGB)"