Implement conditions on tag

A condition is formed by:
    <tag> <op> <value>

<tag> is the normal yambar tag. <op> is one of '==', '!=', '<', '<=', '>', or
'>='. <value> is what you wish to compare it to.

'boolean' tags must be used directly. They falsehood is matched with '~':

    <tag>
    ~<tag>

Finally, to match an empty string, one must use ' "" ':
    <tag> <op> ""

doc/yambar-modules-removables.5.scd

@@ -74,13 +74,12 @@ bar:
     - removables:
         content:
           map:
-            tag: mounted
-            values:
-              false:
+            conditions:
+              (mounted == false):
                 string:
                   on-click: udisksctl mount -b {device}
                   text: "{label}"
-              true:
+              (mounted == true):
                 string:
                   on-click: udisksctl unmount -b {device}
                   text: "{label}"