Xmoji: Fix font settings for tooltips

There was already a resource `tooltipFont` implemented, but not
documented. For fallback, it uses the class name (`Font`), but this
creates a problem because it was also used for the `emojiFont`, which
should almost always be a different font. Therefore:

* add a `classname` property to `FontOptions`, and use this in
  Widget_createFontResName(), so we can override the class name for the
  emoji font.
* Use class `EmojiFont` when creating the emoji font in xmoji.c
* Document the `tooltipFont` resource in README.md, together with an
  example using the `Font` resource.

Fixes:	#15

README.md

@@ -80,7 +80,9 @@ individual widgets.
 
 * `font` (class `Font`): The font for display of normal text. Default:
   `sans`.
-* `emojiFont` (class `Font`): The font for display of emojis. Default:
+* `tooltipFont` (class `Font`): The font for display of tooltips. Default:
+  `sans`.
+* `emojiFont` (class `EmojiFont`): The font for display of emojis. Default:
   `emoji`.
 
 ### Generic rendering options
@@ -156,10 +158,12 @@ names. The following colors are available:
 
 ### Example
 
-The following X resources configure a different emoji font, a slightly larger
-scroll bar, a dark color scheme and a shorter delay before displaying emoji
-names (by scoping the value only to EmojiButton):
+The following X resources configure a slightly smaller default font (for
+normal text and tooltips by specifying the class name), a different emoji
+font, a slightly larger scroll bar, a dark color scheme and a shorter delay
+before displaying emoji names (by scoping the value only to EmojiButton):
 
+    Xmoji*Font: sans-10
     Xmoji*emojiFont: Twitter Color Emoji
     Xmoji*Foreground: #c8c6c5
     Xmoji*Background: #211f1d

src/bin/xmoji/font.h

@@ -27,6 +27,7 @@ typedef enum FontGlyphType
 
 typedef struct FontOptions
 {
+    const char *classname;
     float maxUnscaledDeviation;
     uint8_t pixelFractionBits;
 } FontOptions;

src/bin/xmoji/widget.c

@@ -260,7 +260,8 @@ Font *Widget_createFontResName(void *self, const char *name,
 {
     if (!name) name = "font";
     XRdb *rdb = X11Adapter_resources();
-    XRdb_register(rdb, "Font", name);
+    XRdb_register(rdb, options && options->classname
+	    ? options->classname : "Font", name);
     const char *pattern = XRdb_value(rdb,
 	    XRdbKey(Widget_resname(self), name), XRQF_OVERRIDES);
     char *reqpat = 0;

src/bin/xmoji/xmoji.c

@@ -501,7 +501,13 @@ static int startup(void *app)
 	    Config_injectorFlags(self->config));
 
     /* Create emoji font */
-    self->emojiFont = Widget_createFontResName(win, "emojiFont", "emoji", 0);
+    FontOptions options = {
+	.classname = "EmojiFont",
+	.maxUnscaledDeviation = 0,
+	.pixelFractionBits = (uint8_t)-1
+    };
+    self->emojiFont = Widget_createFontResName(win,
+	    "emojiFont", "emoji", &options);
     ConfigChangedEventArgs ea = { 0 };
     onscalechanged(self, 0, &ea);