Bring back cute mode with hearts for battery segment

[xx4h]

After reviewing the in #435 mentioned commit (cab0747), it looks like all the changes were intentional.

Before that commit the two modes configured through TMUX_POWERLINE_SEG_BATTERY_TYPE:

• percentage
• cute
  were either displaying the current battery status in percentage, or by a number of hearts (cute).

The commit you mentioned has changed basically changed the looks of cute from hearts to a battery symbol.
So in general, nothing "broke", there's now just a new design for cute.

But to bring back the hearts, we should extend the possible values for TMUX_POWERLINE_SEG_BATTERY_TYPE by something like battery or symbol, to then have:

• percentage (showing battery status in percentage)
• symbol (showing battery status with battery symbol)
• cute (showing battery status with as many hearts as defined in TMUX_POWERLINE_SEG_BATTERY_NUM_HEARTS )

[xx4h]

Fixed with #447