Nbgl on nano fixes

[iartemov-ledger]

Addressing https://ledgerhq.atlassian.net/browse/B2CA-2132?focusedCommentId=553816 namely:

• icons redesign follow-up (Home scree: white background and the same as on Dash Board; Operation: black screen/inversed)
• More detailed "Loading transaction" and "Loading message" instead of generic "Processing"
• Using SDK-provided app_exit()
• Fixing the return to Home menu forgotten in Nbgl migration for Nano devices #325
• On transaction using LL Nano screen freeze for some time until finally the "Processing" appears (https://ledgerhq.atlassian.net/browse/B2CA-2132?focusedCommentId=553816)=> it appears to exists before the NBGL migration - let's consider it separately
• Messages indexes restart on each chunk using nbgl_useCaseReviewStreamingContinue() => we should fix it probably by removing chunking and streaming, as separate change-set

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.69%. Comparing base (f644b21) to head (9d72e39).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #328   +/-   ##
========================================
  Coverage    84.69%   84.69%           
========================================
  Files           17       17           
  Lines         2228     2228           
========================================
  Hits          1887     1887           
  Misses         341      341           

Flag	Coverage Δ
unittests	84.69% <ø> (ø)

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[bigspider]

LGTM, only minor comments