Merge 2.4 into master

Original pull requests:

* #2084 from ilya-lavrenov:cont
* #2086 from ilya-lavrenov:mul_fix
* #2087 from ilya-lavrenov:remap_sse2_cond
* #2092 from alalek:fix_mingw_warn
* #2094 from SpecLad:multimon-be-gone
* #2106 from robbertkl:patch-2
* #2108 from pemmanuelviel:flannMemoryLeak
* #2114 from kazuki-ma:SparseMat_convertTo_typofix_24
* #2115 from nghiaho12:kmeans_sample
* #2118 from ilya-lavrenov:semicolons
* #2132 from ComFreek:patch-2
* #2137 from ilya-lavrenov:gitignore