Fix setting default values for configuration entries.

Previously, one would check the value in &confInt["someKey"] by passing a
reference to it to evalIntConf. However, because this passing of the reference
went through std::unordered_map::operator[], it created a slot with the named
key and initialised its value with the default constructor of int, which
placed 0 there, if it didn't exist. If the value of 0 was acceptable for the
setting, then 0 as the value selected by the user was indistinguishable from
a slot that had been just created and had to be set to its default.

Now, the std::unordered_map is passed along with the key so that evalIntConf
can check whether the key exists and, if it doesn't exist, set the value to
its default.
Make the button repeat rate (after the first repetition) a user setting.

The setting's unit is repetitions per second. Its default value is set to
10, and anything between 0 (disabled) and 20 (50 ms) is acceptable.