New keycode macro (XP) for shifted character pairs using UNICODEMAP + bug fixes and improvements (#4803)

* Expose unicode_saved_mods

* Add UNICODEMAP shift pair functionality and XS keycode

* Add XS to keycode reference documentation

* Pick pair index based on both Shift and Caps Lock state

* Add XS to Unicode feature docs

* Clean up process_unicode* headers

* Extract unicode_map index calculation into function

* Pick pair index as XOR rather than OR of Shift and Caps states

* unicode_input_start() has to be called before the unicode_map index is calculated

* Replace unicodemap_input_error() with more generic unicode_input_cancel()

* Replace register+tap+unregister with tap_code16(LCTL(LSFT(KC_U)))

* UNICODE_OSX_KEY → UNICODE_KEY_OSX, UNICODE_WINC_KEY → UNICODE_KEY_WINC

* Make keycode range checks more robust

* Fix keycode range checks for different input modes

* Add UNICODE_KEY_LNX, update docs

* QK_UNICODEMAP_SHIFT → QK_UNICODEMAP_PAIR

* XS → XP, update docs

* Tweak Unicode docs

* Use recently added MOD_MASK_SHIFT and IS_HOST_LED_ON helpers

* Update Unicode table in docs/keycodes.md

* Update Unicode docs per review comments

* Replace references to Mac OS X with macOS in Unicode docs

* As of v0.9.0, WinCompose supports all possible code points

* Expand descriptions in XP docs

* Update keycode table and cycling docs

* Further expand cycling docs

1 files changed, 5 insertions, 2 deletions

diff --git a/quantum/quantum_keycodes.h b/quantum/quantum_keycodes.h
index 19bd7c2165..779c355efc 100644
--- a/quantum/quantum_keycodes.h
+++ b/quantum/quantum_keycodes.h
@@ -87,7 +87,9 @@ enum quantum_keycodes {
 #endif
 #ifdef UNICODEMAP_ENABLE
     QK_UNICODEMAP         = 0x8000,
-    QK_UNICODEMAP_MAX     = 0x83FF,
+    QK_UNICODEMAP_MAX     = 0xBFFF,
+    QK_UNICODEMAP_PAIR    = 0xC000,
+    QK_UNICODEMAP_PAIR_MAX = 0xFFFF,
 #endif
 
     // Loose keycodes - to be used directly
@@ -712,7 +714,8 @@ enum quantum_keycodes {
 #endif
 #ifdef UNICODEMAP_ENABLE
   // Allows Unicode input up to 0x10FFFF, requires unicode_map
-  #define X(i) (QK_UNICODEMAP | (i))
+  #define X(i)     (QK_UNICODEMAP | (i))
+  #define XP(i, j) (QK_UNICODEMAP_PAIR | ((i) & 0x7F) | (((j) & 0x7F) << 7)) // 127 max i and j
 #endif
 
 #define UC_MOD  UNICODE_MODE_FORWARD