Make it possible to use select keys like F1, F2, F3 ...

Select keys like F1, F2, F3, ... are necessary for the ipa-x-sampa
table which has:

VALID_INPUT_CHARS == +!\%&'-./0123456789:<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[]^_`abcdefghijklmnopqrstuvwxyz{|}~"

Therefore, using 1, 2, 3, ... as the select keys for ipa-x-sampa
is bad because it makes it impossible to input the following
entries of the ipa-x-sampa table:

    1	ɨ	0	### U+0268 LATIN SMALL LETTER I WITH STROKE
    2	ø	0	### U+00F8 LATIN SMALL LETTER O WITH STROKE
    3	ɜ	0	### U+025C LATIN SMALL LETTER REVERSED OPEN E
    3\	ɞ	0	### U+025E LATIN SMALL LETTER CLOSED REVERSED OPEN E
    4	ɾ	0	### U+027E LATIN SMALL LETTER R WITH FISHHOOK
    5	ɫ	0	### U+026B LATIN SMALL LETTER L WITH MIDDLE TILDE
    6	ɐ	0	### U+0250 LATIN SMALL LETTER TURNED A
    7	ɤ	0	### U+0264 LATIN SMALL LETTER RAMS HORN
    8	ɵ	0	### U+0275 LATIN SMALL LETTER BARRED O
    9	œ	0	### U+0153 LATIN SMALL LIGATURE OE

    _0	̥	0	### U+0325 COMBINING RING BELOW
    _1	₁	0	### U+2081 SUBSCRIPT ONE
    _2	₂	0	### U+2082 SUBSCRIPT TWO
    _3	₃	0	### U+2083 SUBSCRIPT THREE
    _4	₄	0	### U+2084 SUBSCRIPT FOUR
    _5	₅	0	### U+2085 SUBSCRIPT FIVE
    _6	₆	0	### U+2086 SUBSCRIPT SIX

Also, use the key codes of the select keys instead of the names of the
select keys in the key even processing. One could of course keep
using the names and still make using select keys like F1, F2, F3, ...
possible. But other similar key lists like self._commit_keys,
self._page_up_keys, and self._page_down_keys use key codes, it seems
nicer make all these similar key lists use the same data types.