Upgrade targeted version of Tcl / Tk to v8.6.10

[AndrejMitrovic]

Master currently targets v8.6.0.

[AndrejMitrovic]

Interesting notes in the changelog:

v8.6.1

• Tcl_NR*Eval*() routines now schedule only, as documented.

It seems there exist some more performant / minimum stack space Tcl evaluation routines:

• https://core.tcl-lang.org/tips/doc/trunk/tip/322.md
• https://wiki.tcl-lang.org/page/NRE

This could be useful for enhancing performance.

v8.6.3

• New Windows file dialogs:

• https://core.tcl-lang.org/tips/doc/trunk/tip/432.md

Should check out if there's anything extra we need to do to support them, or if they're enabled by default.

• Adds Windows 10 support

v8.6.4

• Tk-Cocoa 2.0

Should mean better-looking GUIs on the Mac.

• [TIP 433] New binding substitution %M

Could be interesting. See https://core.tcl-lang.org/tips/doc/trunk/tip/433.md

v8.6.5

• [TIP 441] New option [listbox ... -justify].

New GUI option. See https://core.tcl-lang.org/tips/doc/trunk/tip/441.md

v8.6.6

• [TIP 443] More configuration options for text tags.
  New text GUI element options. See https://core.tcl-lang.org/tips/doc/trunk/tip/443.md

• [TIP 446] [$text edit (canundo|canredo)].

New undo options for the text edit widgets. See https://core.tcl-lang.org/tips/doc/trunk/tip/446.md

v8.6.7

• [TIP 464] Support multimedia keys on Windows

This should be very useful to us. Enables building media players, for example. See https://core.tcl-lang.org/tips/doc/trunk/tip/464.md

v8.6.8

• Drop support for macOS 10.5

v8.6.9

• Mac OSX 10.14 (Mojave) support

• [ttk::scale] has new option -state

Seems like a new GUI feature.

v8.6.10

• [tk::unsupported::MacWindowStyle isdark] for Dark Mode,

• support for Tabbed Windows

• Tk improvements on Windows
  - horizontal scrolling support
  - support for buttons 4 and 5
  - new keycodes Mail, AudioMedia, Launch0, Launch1

• [TIP 527] New command [tcl::unsupported::timerate] for performance testing.
  See https://core.tcl-lang.org/tips/doc/trunk/tip/527.md

• Partial support for emoji in Tk text displays (see Tk demo).

• Refactored all MouseWheel bindings

• This might break existing bindings.

• [TIP 535] Revised precision of ::scale widget tick mark values

• This might break existing bindings.
  See https://core.tcl-lang.org/tips/doc/trunk/tip/535.md

• [TIP 533] New index argument in [$menubutton post x y index]
  See https://core.tcl-lang.org/tips/doc/trunk/tip/533.md